U.S. Pat. No. 5,172,044 describes a charging method related to a secondary battery. In this method, the secondary battery is first charged with a constant current, until the terminal voltage of the secondary battery reaches a predetermined voltage value. After that, the secondary battery is charged with a constant voltage of a predetermined voltage value to prevent the battery from being overcharged by an overvoltage. To reduce charging time, a larger current value may be selected during the constant current charging interval. However, the charging current should be limited to a current value which does not excessively deteriorate the charging properties of the secondary battery.
Japanese Patent Laid-open No. HEI 2-119539 also describes a charging method in which a secondary battery is charged with a constant current until the terminal voltage of the secondary battery reaches a first predetermined voltage value. However, the secondary battery is then charged with a second predetermined constant voltage value which is lower than the first predetermined voltage value. In this method, charging the secondary battery with a constant current to the higher first predetermined voltage value, can reduce charging time. However, if the first predetermined voltage value is set excessively high, the charging properties of the secondary battery can deteriorate excessively.
To address the above problems, one of the inventors of the present invention developed a charging method which, as described in Japanese Patent Laid-open No. HEI 6-113474, charging is started and stopped alternately to charge the battery with pulses. This charging method reduces charging time without causing excessive deterioration by overcharging the secondary battery. In this method, at the beginning of charging, the secondary battery is first charged with a constant current until the terminal voltage of the secondary battery reaches a first predetermined voltage value. After that, the secondary battery is pulse charged with pulses at a constant voltage of the first predetermined voltage value. After the pulse charging, the secondary battery is charged with a constant voltage value of a second predetermined voltage value which is lower than the first predetermined voltage value.
In this method, during the pulse charging, the secondary battery is charged with the first predetermined voltage value higher than the second predetermined voltage value. It has been found that pulse charging in which charging is started and stopped alternately, reduces deterioration of the secondary battery caused by an overcharge, as compared to continuous charging.
This charging method has the following disadvantages. In this charging method, the secondary battery is first charged with a constant current, then with pulses and finally with a constant voltage. Therefore, the charging circuit for this method can be complicated. In addition, during the pulse charging interval, the secondary battery is charged with the high first predetermined voltage value for the same period in each pulse. As charging progresses toward a full charge level, charging for the same duration in each pulse can cause deterioration of the charging property of the secondary battery by an overcharge.